(1) Field of the Invention
This invention generally relates to a system and apparatus to identify a position and near instantaneous track of an underwater object moving at any subsonic speed.
More particularly, the invention relates to a system and apparatus as described in which the position of the underwater object is determined by a reflection of a radiated sound field from at least one projectile interacting with the underwater object. The system consists of a multi-barreled gun that discharges a multiplicity of projectiles in a predetermined time sequence and/or pattern. The projectiles themselves are designed to travel at a significant range and are modified to produce a high amplitude narrow frequency radiated noise. The entire system is coupled with a sonar tracking system and a trainable launcher. The preferred embodiment is described in connection with a submarine based gun and targeting system.
(2) Description of the Prior Art
The prior art for underwater targeting systems rely on the projection of an active sonar pulse or “ping” P1 and the received return signal RP1 from a target 90 such as that shown in FIG. 1A. The return signal RP1 is processed to determine the location of the underwater target 90. In the system of FIG. 1A, the sound ping P1 leaves an acoustic transmitter T1. The underwater object 90 is located at a distance d from a platform 92. The ping P1 as reflected off the target 90 becomes reflected ping RP1, travels an approximate distance d back toward the platform 92 and is intercepted by a sonar receiver S1. The information from the sonar receiver S1 is processed. A command is sent to a sonar array (not shown) to transmit a new ping P2. Ideally, the new ping P2 is directed in the relative direction of travel of the underwater object 90 and the platform 92. The time delay in this system is (2*d/c) where c is the sound speed of the environment. The determination of the location of the underwater object depends on the ability to transmit, receive and process the sonar information. The signal processing in general is more effective when high relative Doppler shift exists between the underwater object 90 and the background acoustic environment and the propagation distance is minimized. This system has been used successfully for target tracking and underwater imaging since World War II. The processing of the return signal RP1 may be complicated by a number of factors such as bending of acoustic waves in thermal or salinity gradients, multiple target reflection and the like. For torpedo defense application, both precise instantaneous target position and estimate of target track are critical to system performance. The system performance could be greatly improved with a more accurate tracking system.
Another known solution to the targeting problem is shown in FIG. 1B. In the device of FIG. 1B, the ping P1 is replaced by a projectile path P(path). The projectile P travels toward the underwater object 90 at high speeds and ideally would travel just below sonic speed. The projectile P is assumed to be a compact noise source. As the projectile P approaches the target 90, the radiated noise from the projectile P at point p will interact with the object 90. The amplitude of the reflected sound will be a function of both the amplitude of the projectile's radiated sound and the miss distance Md. This reflected sound S1 then travels back to the platform 92 and is processed. This system yields more accurate information than the system of FIG. 1A because the position of the projectile P in space is known. The high rate of change in the Doppler return as the projectile P approaches and then passes the target 90 allows calculation of the miss distance. The processing of data in the FIG. 1B system relies on the knowledge of projectile position (this is a known quantity with an error associated with projectile dispersion) and sufficient signal to noise ratio and a prior understanding of the spectrum of the radiated noise of the projectile. The prior art does not teach an ideal projectile for this use.
Another concern for overall targeting is the determination of the object track. In general, three to five separate pings of data are required to determine the track of object 90. FIG. 2 shows that for either of the known approaches in FIG. 1A and FIG. 1B, the target 90 will change position during the time between the transmission of the first ping or projectile and the processing of the fourth return. The distance the object migrates (Tmd) during that time in comparison to the initial standoff distance d is an important source of error in the understanding of the object's true position and track. The prior art does not address this problem.
The following patents, for example, disclose object detection systems, but do not disclose the determination of a position of an underwater object by reflection of a radiated sound field pattern from projectiles interacting with the underwater object.
U.S. Pat. No. 4,350,881 to Knight et al.;
U.S. Pat. No. 5,062,641 to Poillon et al.;
U.S. Pat. No. 5,481,505 to Donald et al.;
U.S. Pat. No. 5,614,657 to Harada;
U.S. Pat. No. 5,929,370 to Brown et al.; and
U.S. Pat. No. 6,405,653 to Miskelly.
Specifically, Knight et al. disclose an apparatus for indicating the location in a measurement plane through which a projectile passes. The apparatus includes an array of at least three transducers responsive to the airborne pressure wave produced by the projectile and positioned at predetermined locations along a line parallel to the movement plane. The apparatus further includes a device for measuring the velocity of the projectile and another for measuring the velocity of sound in air in the vicinity of the transducers. A computing means, responsive to the array of transducers, the velocity measuring means and the propagation of sound determination is provided which determines the location in the measurement plane through which the projectile passed and provides an output indicating that location. Also disclosed is a means, in combination with the position detecting means, for detecting and providing a positive indication of a projectile hit on a target member.
The patent to Poillon et al. discloses a system that accurately determines the location of the point of impact of a projectile, such as a golf ball, on a screen. A timer is provided that is activated when the projectile leaves a start point positioned at a known location. Upon impact of the projectile on the screen, a sound wave is produced that travels to a plurality of sound wave detectors. The system measures the time the sound wave travels to the plurality of sound wave detectors. These travel times are utilized to determine the point of impact of the projectile on the screen. The point of impact and flight travel time is used to determine the trajectory and velocity of the projectile. These parameters are then used to determine the distance the projectile would have traveled if unimpeded.
Donald et al. disclose a method and apparatus for detecting, processing and tracking sonar signals to provide bearing, range and depth information that locates an object in three-dimensional underwater space. An inverse beamformer utilizes signals from a towed horizontal array of hydrophones to estimate a bearing to a possible object. A matched field processor receives measured covariance matrix data based upon signals from the hydrophones and signals from a propagation model. An eight nearest neighbor peak picker provides plane wave peaks in response to output beam levels from the matched peaks within the specified limit of frequency, bearing change over time, range and depth to specify an object as a target and to display its relative range and depth with respect to the array of hydrophones.
Harada discloses a three dimensional measuring apparatus including a gun, a pedestal for the gun, three microphones, a pedestal of a product and a data processor. The gun shoots very small bullets at a target such as a casting product. The very small bullets explode or rupture and generate high frequency sound at the time of hitting against the target. The three microphones catch the high frequency sounds. All these sound data are gathered to the data processor, and processed into three-dimensional data of the surface of the product.
The patent to Brown et al. discloses a projectile propelled from a location in air, through an air/water interface, and toward a submerged underwater object. The projectile includes a forward end that forms a cavitation void around the projectile in water, avoiding water drag on the remainder of the projectile. The projectile further includes an outwardly flared or finned rearward end that aerodynamically stabilizes the projectile in air and flare stabilizes it in water, in each case against yaw.
Miskelly discloses a supercavitating underwater projectile adapted to be fired from a gun or the like, comprising a front end or nose portion and a rear end portion. An auxiliary rocket motor is disposed within the rear end portion of the projectile for providing additional thrust after the projectile has been fired. Vents are disposed within the projectile and are in communication with the rocket motor and the exterior of the projectile for venting some of the combustion gases from the rocket motor to the exterior of the projectile near the nose portion thereof to increase the size of the cavitation bubble formed as the projectile travels through the water and thereby reduce hydrodynamic drag on the projectile.
It should be understood that the present invention would in fact enhance the functionality of the above patents by providing better resolution of the position of underwater objects, improved determination of the track of an underwater object, the ability to more effectively target underwater objects moving at high speed, better resolution of underwater objects and tracks in poor acoustic environments, decreased signal processing requirements to achieve a desired target resolution, and better ability to resolve multiple targets.